Stepper motors are typically controlled based on sensed individual phase currents of the controlled stepper motor. A controller receives the sensed phase currents from phase current sensors located on each phase of the stepper motor. The controller then communicates with, and controls, the stepper motor using an H-bridge interface. The stepper motor interface can be arranged as either a unipolar interface or a bipolar (two phase) H-bridge interface, with the arrangement depending on the particular stepper motor. The H-bridge interface is controlled via a controller that operates multiple switches within the H-bridge interface, thereby causing the H-bridge interface to output proper stepper motor control signals.
In order to introduce redundancy and protect against a failure in the stepper motor controls, some stepper motor implementations use two or more stepper motor control channels to provide control signals to the stepper motor. In configurations using multiple stepper motor control channels, only a single control channel can provide control signals to the stepper motor at a time. In order to isolate the inactive stepper motor control channel from the stepper motor, and thereby prevent both controllers from transmitting control signals to the stepper motor simultaneously, a mechanical relay is used to physically disconnect one control channel from the stepper motor and connect the other control channel to the stepper motor.